Issue in std::basic_string<charT> to_simple_string_type(time_duration td) with locales

There is a problem std::basic_string to_simple_string_type(time_duration td) if application doesn't use the locale("C").

If you use spanish locale the result of this function is like this one:
2020-11-03T09:26:04.966.395
where the second 'dot' is wrong because is the miles signal in spanish locale.

I propose to correct it, use next code:
```
std::basic_ostringstream ss;
ss.imbue(std::locale::classic());

```

Best regards